Deployments | Kubernetes

A Deployment provides declarative updates for Pods and ReplicaSets.
You describe a desired state in a Deployment, and the Deployment Controller changes the actual state to the desired state at a controlled rate. You can define Deployments to create new ReplicaSets, or to remove existing Deployments and adopt all their resources with new Deployments.
The following are typical use cases for Deployments:
The following is an example of a Deployment. It creates a ReplicaSet to bring up three nginx Pods:
A Deployment named nginx-deployment is created, indicated by the .metadata.name field. This name will become the basis for the ReplicaSets and Pods which are created later. See Writing a Deployment Spec for more details.
The Deployment creates a ReplicaSet that creates three replicated Pods, indicated by the .spec.replicas field.
The .spec.selector field defines how the created ReplicaSet finds which Pods to manage. In this case, you select a label that is defined in the Pod template (app: nginx). However, more sophisticated selection rules are possible, as long as the Pod template itself satisfies the rule.
The .spec.template field contains the following sub-fields:
Before you begin, make sure your Kubernetes cluster is up and running. Follow the steps given below to create the above Deployment:
Create the Deployment by running the following command:
Run kubectl get deployments to check if the Deployment was created.
If the Deployment is still being created, the output is similar to the following:
When you inspect the Deployments in your cluster, the following fields are displayed:
Notice how the number of desired replicas is 3 according to .spec.replicas field.
To see the Deployment rollout status, run kubectl rollout status deployment/nginx-deployment.
The output is similar to:
Run the kubectl get deployments again a few seconds later. The output is similar to this:
Notice that the Deployment has created all three replicas, and all replicas are up-to-date (they contain the latest Pod template) and available.
To see the ReplicaSet (rs) created by the Deployment, run kubectl get rs. The output is similar to this:
ReplicaSet output shows the following fields:
Notice that the name of the ReplicaSet is always formatted as [DEPLOYMENT-NAME]-[HASH]. This name will become the basis for the Pods which are created.
The HASH string is the same as the pod-template-hash label on the ReplicaSet.
To see the labels automatically generated for each Pod, run kubectl get pods --show-labels. The output is similar to:
The created ReplicaSet ensures that there are three nginx Pods.
You must specify an appropriate selector and Pod template labels in a Deployment (in this case, app: nginx).
Do not overlap labels or selectors with other controllers (including other Deployments and StatefulSets). Kubernetes doesn't stop you from overlapping, and if multiple controllers have overlapping selectors those controllers might conflict and behave unexpectedly.
The pod-template-hash label is added by the Deployment controller to every ReplicaSet that a Deployment creates or adopts.
This label ensures that child ReplicaSets of a Deployment do not overlap. It is generated by hashing the PodTemplate of the ReplicaSet and using the resulting hash as the label value that is added to the ReplicaSet selector, Pod template labels, and in any existing Pods that the ReplicaSet might have.
Follow the steps given below to update your Deployment:
Let's update the nginx Pods to use the nginx:1.16.1 image instead of the nginx:1.14.2 image.
or use the following command:
where deployment/nginx-deployment indicates the Deployment, nginx indicates the Container the update will take place and nginx:1.16.1 indicates the new image and its tag.
The output is similar to:
Alternatively, you can edit the Deployment and change .spec.template.spec.containers[0].image from nginx:1.14.2 to nginx:1.16.1:
The output is similar to:
To see the rollout status, run:
The output is similar to this:
Get more details on your updated Deployment:
After the rollout succeeds, you can view the Deployment by running kubectl get deployments. The output is similar to this:
Run kubectl get rs to see that the Deployment updated the Pods by creating a new ReplicaSet and scaling it up to 3 replicas, as well as scaling down the old ReplicaSet to 0 replicas.